The vermilion locus of Drosophila encodes an enzyme, tryptophan oxygenase, which catalyzes one of the first steps in the synthesis of the brown eye pigment. This locus has been well-defined genetically for a number of years. We intend to clone the vermilion locus to study the structure of the gene and control of its expression. We are particularly interested in determining the nature of spontaneous mutations at vermilion that are suppressible by mutations at suppressor of sable; hopefully, these studies will help to elucidate the mechanism by which suppression occurs.